With respect to tires for a vehicle or the like, after manufacturing thereof, a shape inspection of inspecting the presence or absence of a defect by measuring a surface shape such as concavity and convexity or undulation is performed in a tire testing machine. As a geometry measuring device which is used for measurement of the surface shape of such a tire, for example, a geometry measuring device made so as to be able to inspect the presence or absence of a defect of the surface shape such as concavity and convexity or undulation of a tire by measuring the tire with a laser displacement meter or the like while irradiating the tire with a laser beam while rotating a spindle of a tire holding device of a tire testing machine, after the tire is held by a test rim mounted on the spindle, is known (refer to PTL 1 below).
In such a tire shape measuring device, if the above-described inspection is performed by a specified number of times or for a specified period of time in order to maintain measurement accuracy, confirmation or correction of the measurement accuracy is performed by mounting a tire-simulated master disk instead of the test rim on the spindle and measuring the surface shape of the master disk.